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I purchased some of these cells used and want to test the individual capacities before I build them up as a pack. The spec says that they are rated at 180ah @ .3C

So that would be 180 * .3 for a 54 amp load. Can I just use a power resistor (around .05925 ohms) to achieve the 54 amp load and count then time until the cell hits 2.5 volts? Should be around 3 1/3 hours. Do I need to be taking into account that the voltage will be dropping during this test? Should I be counting watt hours instead?

I know I could use a amp meter, but I want to understand the principle behind the test. Thanks!
 

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That's as good a way as any unless you have a fancy electronic load that can provide a constant current sink. The bonus to testing is you have drained the cells to a common bottom balance voltage before pack connection.

i use a voltmeter, ammeter and a stopwatch along with a big ass resistor, then monitor and record the data every 6 minutes until the voltage gets near the bottom. Then record every 6 seconds when it breaks the knee and falls--stuff happens really fast at the end. You could quit at 3.0 and not miss what little capacity is down to 2.5, there is none useful down there anyway. You can make a BA power resistor with several stove eyes from the thrift store wired up in parallel.

The tell-all test if you drain the cell that low is whether or not the voltage bounces back up when you release the load. A bad or weak cell won't rebound, or may show some self-discharge, when taken down like that and left open-circuit for a week or so. A good cell will rebound.
 
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